There is a need in mining operations, particularly in longwall mines, for a roof support that yields in a controlled fashion in response to roof loads. In the past, mine operators have used post-type roof supports in the form of wood timbers and cribbing. The timbers absorb the initial roof load and the cribbing yields as load is applied thereto so that catastrophic failure of the roof support is prevented. This type of roof support system is one attempt to approximate an ideal system which would be very stiff to handle the initial load and yet have the ability to yield in a controlled fashion. However, timbers and cribbing present a fire hazard, are expensive and are labor intensive. Also, timbers and cribbing occupy a relatively large volume of space and can thus interfere with ventilation and complicate movement of materials and personnel into and out of a mine.
A yieldable steel post for use in supporting a roof of a mine is disclosed in Coal Magazine, June 1990, pages 136 and 137. The post includes a bottom leg telescoped within an upper leg wherein each leg comprises a length of pipe. A tapered expansion ring is welded on the lower leg at a desired position and bottom and top plates are welded to the lower and upper legs and are adapted to engage a floor and a roof, respectively, of a mine. As a compressive load is applied to the post, the expansion ring radially deforms the upper leg outwardly as the top leg moves relative to the bottom leg.
A prior art yieldable roof support utilizes an upper slotted leg of hollow pipe telescoped within a lower leg also of hollow pipe. A wedge is driven into the slot in the upper leg at a desired position. The wedge causes the upper leg to have an interference fit with the lower leg at this position. As load is applied to the support, the lower leg is radially deformed by the upper section of leg at the location of the wedge.
A further type of yieldable post is sold by Jennmar Corp. of Pittsburgh, Pa. This type of roof support includes an upper channel nested within a lower channel and one or more clamps that couple the upper and lower channels together.
Other types of roof supports include concrete donuts or cribbing and are described in an article appearing in Coal Age, March 1987, pages 30-33.
None of the foregoing post-type roof supports has been found to be entirely satisfactory in terms of expense, ease of manufacture and assembly and support capability.
In addition to post-type supports, anchor-type yieldable roof supports for tunnels and the like have been designed wherein a supporting rod extending through the surface of the rock into the tunnel includes a deformation member carried on one end of the rod in axial alignment with a steel tube. When the rod is placed in tension, the deformation member is deflected within the steel tube, thereby deforming the tube radially outwardly and permitting limited movement of the rod. Such a design is shown in Austrian Patent 378,823.
West German published patent specification 33 14 858 A1 discloses a yieldable roof support including a rod having a series of stepped portions separated by tapered portions on one end thereof disposed within a steel tube. The stepped and tapered sections deform the steel tube radially outwardly when the rod is placed in tension.